GABA.sub.A receptors for .gamma.-aminobutyric acid (GABA) are the most abundant inhibitory receptors in the mammalian brain. The GABA.sub.A receptors are structurally constituted as macromolecular heteropentameric assemblies (combinations of .alpha., .beta., and .gamma./.delta. protein subunits). Several subtypes of such GABA.sub.A receptors have been described by techniques of modern molecular biology.
Each GABA.sub.A receptor complex comprises a chloride ion channel that controls chloride flux across the neuronal membrane, and multiple recognition sites for small modulatory molecules such as benzodiazepines, barbiturates, picrotoxin, and certain steroids. When GABA interacts with its receptor, the ion channel is opened, chloride influx is enhanced, the membrane is hyperpolarized and the cell becomes less responsive to excitatory stimuli. This GABA induced ion current can be regulated by diverse agents, including agents that interact with the benzodiazepine receptor or recognition site.
Agents that bind or interact with the modulatory sites on the GABA.sub.A receptor complex, such as for example the benzodiazepine receptor, can have either enhancing effect on the action of GABA, i.e. a positive modulatory effect of the receptor (agonists, partial agonists), an attenuating effect on the action of GABA, i.e.
negative modulation of the receptor (inverse agonists, partial inverse agonists), or they can block the effect of both agonists and inverse agonists by competitive block (antagonists or ligands without intrinsic activity).
Agonists generally produce muscle relaxant, hypnotic, sedative, anxiolytic, and/or anticonvulsant effects, while inverse agonists produce proconvulsant, anti-inebriant, and anxiogenic effects. Compounds with anxiolytic effects but without or with reduced muscle relaxant, hypnotic and sedative effects are characterised as partial agonists. Partial inverse agonists are considered to be useful as cognition enhancers.
Numerous compounds belonging to different chemical series of compounds having affinity for the benzodiazepine receptors have been synthesized during the last three decades. However, although the benzodiazepine receptor sites are still considered as very attractive biological sites for interfering with the CNS to treat various disorders and diseases, then nearly all previously synthesized compounds acting at these receptor sites have failed during clinical development because of unacceptable side effects.
Benzimidazole compounds for use in treatment of CNS disorders has been described in EP616807; Imidazole compounds as calcium channel blockers are described in EP 563001;